Abundantly available, hydrogen has long been employed in many chemical processes. Now, as industry develops new applications for hydrogen, there is a growing need to store hydrogen safely and conveniently.
Hydrogen is stored conventionally as a gas in steel cylinders at high pressures (e.g., 13.79 MPa or 2,000 psi) and at lower pressures as a liquid in insulated containers. Both methods of storage require comparatively bulky storage containers. In addition to their unwieldy size, such containers are inconvenient due to the high pressure required for gas storage in cylinders and the ever present danger of gaseous hydrogen evolving from boiling-off of the liquid form.
Within recent years, considerable attention has been focused on the storaage of hydrogen as a metallic compound, or hydride, of various substances. Metal hydrides, in the form of powders, can store large amounts of hydrogen at low and even sub-atmospheric pressures in relatively small volumes. This low pressure storage of hydrogen is relatively safe and allows the construction of hydrogen containers having forms significantly different than those presently known.
Apart from the storage of hydrogen, hydrides are also currently being evaluated for gas compression, solar heat storage, heating and refrigeration, hydrogen purification, utility peak-load shaving, deuterium separation, electrodes for electrochemical energy storage, pilotless ignitors and internal combustion engines.
Hydridable metals are charged with hydrogen by introducing pressurized gaseous hydrogen into valved containers. The hydrogen gas reacts exothermically with the metal to form a compound. Discharging of the metal hydride is accomplished by opening the valve of the container, to permit decomposition of the metal hydride, an endothermic reaction. It has been found expedient when gas is desired from the storage vessel to heat the storage vessel thereby increasing the flow of hydrogen or providing hydrogen at pressures substantially above atmospheric.
During the adsorption/desorption process, the hydridable metal has been found to expand and contact as much as 25% in volume as a result of hydrogen introduction and release from the metal lattice. Such dimensional change leads to fracture of the metal powder particles into finer particles. After several such cycles, the fine powder self-compacts causing inefficient heat transfer and, as a consequence, hydrogen transfer. Additionally, and of even greater significance, high stresses due to the compaction of the powder and expansion during hydride formation are directed against the walls of the storage container. The stress within the powder has been observed to accumulate until the yield strength of the container is exceeded whereupon the container plastically deforms, buckles or bulges and eventually ruptures. Such rupture is extremely dangerous since a fine, often pyrophoric powder is violently expelled by a pressurized, flammable hydrogen gas. Small, experimental cylinders of the aforedescribed type have indeed been found to burst when subjected to repetitive charging-discharging conditions.
In particular, the successful application of metal hydride technology for the recovery of usable energy from low grade heat sources such as industrial waste heat and solar energy has been previously hindered by the following problems:
1. Heat transfer through a metal hydride powdered bed is inherently poor. PA0 2. The aforementioned tendencies of the hydrides (due to hydride expansion when absorbing hydrogen) have been known to produce substantial mechanical forces and therefore have hindered the use of economical hydride containment designs. PA0 3. Hydrogen pressure drops through the hydride beds may become so excessive that gas transfer is seriously impaired.
One current solution to the problems enumerated above is the employment of cylindrical capsules to contain the hydride. See U.S. Pat. No. 4,135,621. When oriented in a horizontal position, this technology has been successful in alleviating the deleterious compaction problem. However, poor heat transfer and excessive pressure drops have warranted continued investigation.